class Pen {
			    constructor() {
			        document.onmousedown = this.down.bind(this);
			    }
			    down() {
			        document.onmousemove = this.move.bind(this);
			        document.onmouseup = this.up.bind(this);
			    }
			    move(e) {
			        var e = e || event;
			        var div = document.getElementById("box");
			        div.style.cssText = "left:" + e.clientX + "px; top:" + e.clientY + "px;";
			    }
			    up() {
			        document.onmousemove = null;
			        document.onmouseup = null;
			    }
			}
			
			new Pen();